Danfoss VLT 2800 (Legacy Product) Installation guide

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VLT 2800 Series - Precise Stop
1
MI.28.C1.02 - VLT is a registered Danfoss trademark
Introduction...........................................page 1
Precise Stop .........................................page 1
Typical application .................................page 3-4
Guidelines ..............................................page 4-7
Speed compensated Stop....................page 7
Typical application .................................page 8
Guidelines .............................................page 8-9
Counter Stop ........................................page 9
Guidelines .............................................page 10-11
Calculation of motor size ......................page 12-15
VLT 2800 Series - Precise Stop
MI.28.C1.02 - VLT is a registered Danfoss trademark
2
Precise Stop
Why precise stop?Why precise stop?
Why precise stop?Why precise stop?
Why precise stop?
Packing or filling of containers are often done at high
speed with many start and stop sequences. It is
thus very important that you can rely upon that the
conveyor belt stops exactly where it should to
ensure that the container always has the right
position when assembling, filling etc. If that is not the
case, much waste and downtime of your plant
could be the costly result. If you use a standard
frequency converter the accuracy is determined by
the internal task time. That is not the case when
you use the precise stop function of the VLT 2800; it
eliminates the task time dependence and increases
the accuracy substantially. The precise stop function
is an open loop control.
The frequency converter, however, is not the only
component that limits the system capacity. For the
best possible solution it is important to take a look at
the whole system, both the mechanical part and
the electrical part.
Introduction
This note will give you the information you need to
use the functions of precise stop, speed
compensated stop and counter stop with VLT 2800
frequency converters. This document is to be used
as a suggestion only. Users must ensure that
installations meet national standards and are
suitable for the existing conditions.
The functions described are highly advantageous in
applications requiring high precision, for example in
food and beverage industries.
Examples of advantages:
· High repetition accuracy
· Speed independence
· Cost optimised solution
· Increased flexibility
· Less waste
· Uniform quality
Typical applications and instructions how to use the
functions are described in the following.
This note is intended for personnel familiar with the
VLT 2800 hardware and programming procedures
necessary to operate the VLT 2800. Your local
Danfoss Company shall of course be pleased to give
you any further explanation you may need.
VLT 2800 Series - Precise Stop
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Typical application
Figure 1 Typical use of precise stop function
In Figure 1 a conveyor belt takes a number of boxes
to a filling station. The conveyor belt is driven by an
asynchronous motor controlled by a VLT 2800. The
PLC gets a signal from a proximity sensor when the
next box is on its way. Then the PLC gives a stop
command to the frequency converter, and the next
box is brought forward. In this situation you are not
dependent on a constant distance between the
boxes, there will always be a box under the filler.
Precise placing of the box, however, depends on
the system.
Figure 2 Typical delays at precise stopAll components from the proximity sensor over the
PLC to the frequency converter cause delays. A
proximity sensor typically gives a delay between 0.5
and 10 ms depending on sensor principle and type.
The PLC delay is typically between 1 and 10 ms
depending on program size and structure. If the
delays are constant you can move the proximity
sensor until you have achieved an acceptable result.
However, it is important to consider any delay
tolerance value that should be kept as low as
possible.
VLT 2800 Series - Precise Stop
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4
Example 1:Example 1:
Example 1:Example 1:
Example 1:
A conveyor belt runs at 1.0 m/s. The total delay of
the system has been measured to be 20 ms with a
tolerance of 10%. What is stop length and
accuracy? The belt runs 20 mm from the stop signal
has been given until standstill. This distance can be
adjusted by moving the proximity sensor 20 mm.
The tolerance is 10 % equal to 2 mm. The 2 mm
inaccuracy can only be eliminated by using a
positioning system like a VLT 5000 with Sync/pos
option.
The frequency converter tolerance is normally given
by its task time. For a VLT 2800 it is 13 ms.
However, by using its special precise stop function
the tolerance is independent of the task time
because the stop signal immediately interrupts the
execution of the frequency converter program. The
precise stop function gives a highly reproducible
delay of 20 ms from the stop signal is given until the
ramping down starts.
Figure 3 Ramp inaccuracy
Guidelines
In this section we go through the various factors to
be considered when using precise stop.
Ramps
To achieve the optimal precise stop it is important to
set the ramps correctly. If the ramping down is done
too quickly there will be very few ramp stages
resulting in a relatively high error contribution as
shown in Figure 3. To ensure optimum accuracy
there should be at least 10 cycles during ramping
down. Achievable VLT 2800 expressed in shaft
accuracy in degrees as function of ramp length is
indicated in table 1.
Ramp [ms] 50 150 3000
Accuracy [°] 33 23 9
TT
TT
T
able I able I
able I able I
able I Accuracy at different ramps
The ramps of VLT 2800 refer to the rated motor
speed. Running at other speeds the system can be
further optimised by modifying the ramps according
to actual maximum speed.
Example 2:Example 2:
Example 2:Example 2:
Example 2:
A 50 Hz motor runs at maximum 30 Hz. What must
the ramp of the frequency converter be
programmed for to ensure 10 cycles during ramping
down considering that the task time of the
frequency converter is 13 ms? As from 30 Hz the
ramping down must be done in at least 130 ms, so
the ramping down time of the frequency converter
must be programmed to be at least 5/3*130 = 217
ms.
VLT 2800 Series - Precise Stop
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Figure 5 Position accuracy
Figure 6 Stop accuracy for motor
Tolerances
The tolerance or the accuracy indicates how
precisely the frequency converter can stop the
motor. This is illustrated in Figure 5. In transport
applications the accuracy is normally given by a
length tolerance. For example, the conveyor belt
must stop within ± 2 mm. This accuracy is partly
influenced by the frequency converter’s ability to
stop the motor and partly by the mechanical gear
ratio from motor shaft to conveyor belt. This ratio
can either increase or reduce the inaccuracy. In
most cases, however, the inaccuracy will be
reduced by the gear ratio. See example 3 below.
The shaft position accuracy in degrees gives the
best indication of the basic accuracy of the
frequency converter and motor, as illustrated in
Figure 6. When the precise stop function of VLT
2800 is used the motor shaft accuracy is typically
better than ± 23°. Table II shows the typical
tolerance intervals for a VLT 2800 with precise stop
at ramp times of minimum 150 ms. The tolerance
will decrease with increasing ramp times, see Table I
above. The tolerance intervals are a measure of the
variance. This means that the shaft will in average
stop outside these intervals in 4.56 %, 0.26 % and
0.003 % (30 ppm) of the cases respectively.
Tolerance intervals
2
σ (95.44%) ± 23°
3σ (99.74%) ± 37°
6σ (99.997%) ± 82°
TT
TT
T
able II able II
able II able II
able II Tolerance intervals
If the mechanical gear ratio, and pulley diameter is
known the accuracy can be converted to length
accuracy for the conveyor belt. See example 3
below.
A frequency converter with a task time of 2 ms will
give an inaccuracy of ± 1 ms corresponding to a
theoretical tolerance of ± 9° for a 4 pole
asynchronous motor at rated speed (1500 RPM).
Figure 4 The accuracy is dependent on the
number of samples.
VLT 2800 Series - Precise Stop
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Figure 7 Precise stop using creep speed
Creep speed contra precise stop
In some applications the stop accuracy can be
improved by using a creep speed. In such cases a
proximity sensor is passed when 95% of the
distance is completed. Then the belt is ramped
down to a considerably lower speed, e.g. 5% of
normal speed, then another sensor is passed and
the motor is stopped. This is illustrated in Figure 7.
Figure 7 shows the effect of using creep speed. The
hatched fields of Figure 7 show the stop length at
high and low speeds. The stop length and the
tolerance is much higher when ramping down from
Precise stop inverted versus precise start/stop
With VLT 2800 precise stop can be used in two
different ways. You can either choose Precise stop
inverted or Precise start/stop.
The start signal must be active during the
entire ramping up.
Precise stop inverted:
If terminal 18 or terminal 19 is programmed for
Precise stop inverted you can use a signal source
(sensor or PLC) with positive logic.
Precise start/stop:
If it is important also to have a precise start signal
you choose Precise start/stop. This could for
example be in applications where one movement
must be synchronised with another action, filling
dough on plates /putting cheese on pizzas for
instance. You can also use Precise start/stop if you
want to start and stop the frequency converter with
only one control wire.
full speed than when ramping down from creep
speed. It is thus possible to minimise the influence of
sensor and PLC delay tolerances by using a creep
speed.
The disadvantages of using creep speed are that
two proximity sensors are required and that the total
stop time and length are longer than at direct stop.
This means that the cycle time is increased and that
the efficiency of the machine is reduced. VLT 2800
with precise stop can often make it unnecessary to
use creep speed.
Example 4:Example 4:
Example 4:Example 4:
Example 4:
The conveyor belt is the same as that used in
example 1. In example 4 a creep speed is used that
is 5% of rated speed. What is the stop length and
what is the accuracy? After having passed sensor 1
the belt runs 0.05 m/s. With a delay of 20 ms ±
10% this gives a stop length of 1 mm and a
tolerance of 0.1 mm. The stop length from the first
sensor is not considered. – Note the considerable
improvement compared to example 1.
Example 3:Example 3:
Example 3:Example 3:
Example 3:
A VLT 2800 is connected to a gear motor with a
gear ratio of 3:1. The gear is connected to a
conveyor belt running 100 mm per revolution. How
precisely can this conveyor belt be stopped at 3
σ
accuracy? The ± 37° from tabel II is converted to
length accuracy as follows:
37/360 x 1/3 x 100 mm = 3.5 mm.
This means that the belt will stop within ± 3.5 mm in
99.74 % of all cases.
VLT 2800 Series - Precise Stop
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Figure 8 Relevant DC braking parameters
Braking
Dynamic braking:
When braking loads of high inertia are used it may
be necessary to use a brake resistor with the
frequency converter. This makes it possible to
dissipate the surplus energy that arises during
braking. For further information about brake resistor
applications we refer to instruction MI.50.D2.
DC braking:
To optimise precision you can use DC- braking. This
function is integrated in the frequency converter. As
a DC voltage is applied to one motor winding there
will be a brake torque making it possible to brake
the motor at low speed (below 8% of rated speed).
The parameters to be programmed are shown in
Figure 8.
Using this function the conveyor belt can be brought
to standstill without help from the friction of the
application. This means higher accuracy, especially if
the natural friction is low.
Figure 9 Speed compensated stop
Speed compensated stopSpeed compensated stop
Speed compensated stopSpeed compensated stop
Speed compensated stop
Why speed compensated stop?Why speed compensated stop?
Why speed compensated stop?Why speed compensated stop?
Why speed compensated stop?
Precise stop is only optimal when the operational
speed of the conveyor belt is constant. If you use
the same line for different products you will often
have to change the belt speed. In such cases you
cannot rely on the precise stop function to ensure
that work pieces are placed correctly. The reason is
that the stop length varies with the original speed.
To solve this problem it is necessary to compensate
for the actual speed. With VLT 2800 that is done by
using the function speed compensated stop. At
speeds below maximum speed the stop signal is
delayed internally before speed is reduced by the
programmed ramp. This is illustrated in Figure 9
where the stop length is the same at the two
speeds if the size of area 1 is the same as the sum
of areas 2 and 3. Speed compensated stop utilises
the precise stop function of VLT 2800.
122 Function at stop
(FUNCTION AT STOP)
123 Min.frequency for activation of function
at stop
(MIN.F.F.FUNC.STOP)
126 DC brake time
(DC BRAKING TIME)
127 DC brake cut-in frequency
(DC BRAKE CUT-IN)
132 DC brake voltage
(DC BRAKE VOLTAGE)
VLT 2800 Series - Precise Stop
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TT
TT
T
ypical applicationypical application
ypical applicationypical application
ypical application
Figure 10 shows a typical speed compensated stop
application. Dough is placed on the conveyor belt, a
belt takes the dough to a belt oven and from the
oven the belt leads the cakes through the packing
procedure. A flexible production plant that can
handle different types of products requires speed
changes of the conveyor belt. The time in the oven
depends for example of the cake size and type and
therefore the belt speed must be adjusted
accordingly. Without speed compensated stop the
distance between the cakes and capacity of the
plant will vary from cake type to cake type and it will
be necessary to move or adjust the proximity sensor
mechanically. Such mechanical adjustments reduce
the efficiency and flexibility of the plant. The speed
compensated stop function ensures that the
distance between the cakes is the same at all
speeds.
Figure 10 Typical speed compensated stop
application
GuidelinesGuidelines
GuidelinesGuidelines
Guidelines
In this section we go through the factors that must
be considered for speed compensated stop
applications. Please also consider the factors
mentioned under precise stop.
Speed reference
The delay is calculated on the basis of the reference
speed of the frequency converter and not on the
basis of the actual speed. Please therefore make
sure that the frequency converter has ramped up
before you activate the speed compensated stop.
The ramp delay is calculated relative to the
maximum output frequency programmed in
parameter 202.
Setting of ramp up time
The ramp up time may affect the accuracy of speed
compensated stop. Much too short ramps may
result in overshoot or ringing of the mechanical
system, as shown in Figure 11, or current limit in the
frequency converter. Such instability will of course
give rise to high inaccuracy. Increasing the ramp up
time can reduce overshoot and the risk of
resonance.
Figure 11 The ramp time is too short
VLT 2800 Series - Precise Stop
9
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If the frequency converter runs into current limit it
will try to increase the ramp time and this can have
the consequence that the belt has not reached the
desired speed when ramping down, and thus
introduce an inaccuracy. Appendix A gives more
details about this and a way to calculate the
required power as a function of the ramp time.
Sensor and PLC delay
The frequency converter can only compensate for
the inaccuracy that arises because of different stop
lengths during ramping down. Before the frequency
converter starts ramping down the belt runs a
distance, how long depends on the speed. The time
depends on the delay of the sensor and the PLC,
before the braking starts. To compensate for this
you can program parameter 349 for a total delay in
ms enabling the frequency converter to compensate
for this delay as well.
Speed compensated stop for commissioning
Commissioning is often done at a lower speed for
safety reasons. This makes it easier to see what is
going on. However, in some applications, for
example where the distance to be done must be
precise, it is not directly possible to use a lower
speed. In such cases the speed compensated stop
could be of great help by making commissioning
possible at low speed.
Counter stopCounter stop
Counter stopCounter stop
Counter stop
Why counter stop?Why counter stop?
Why counter stop?Why counter stop?
Why counter stop?
The VLT 2800 frequency converter can be
controlled via a pulse train. The frequency converter
counts the number of pulses, typically from an
encoder and generates a stop signal after a pre-
programmed number of pulses. The advantage of
using counter stop is that you get a direct feedback
from the process, for example from the conveyor
belt. In this case backlash and other mechanical
variations will not affect the accuracy. The control is
done in a closed loop. This is a one-way control
meaning that the motor will not be reversed if it
should run too far.
TT
TT
T
ypical applicationypical application
ypical applicationypical application
ypical application
VLT 2800 with counter stop is typically used in
applications requiring strict monitoring of the
distance done. Figure 12 shows an example of such
an application. A frequency converter controls the
feeding forward of paper, plastic foil etc. The
material is automatically cut in fixed lengths. On the
material there is a measuring wheel with an
incremental encoder. The output of the encoder is
connected with the counter input of the VLT 2800
frequency converter. This reduces waste and
makes the quality uniform.
Figure 12 Paper cutter
VLT 2800 Series - Precise Stop
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10
Example 8:
In an application like that shown in Figure 12 a reso-
lution of 0.5 mm is required. The measuring wheel
has a diameter of 100 mm. How many pulses must be
chosen for the incremental encoder? The perimeter
of the measuring wheel is 100 x π = 314 mm. The
number of pulses must therefore at least be: 314/
0.5 = 628 pulses corresponding to an encoder with
1024 pulses/revolution. If the forward speed for the
paper is 1 m/s, what type of output should be
selected? The wheel will rotate 1000/314 = 3.18
revolutions/s. This means that the frequency of the
encoder signal will max be 3.18
.
1024 = 3256 Hz.
An encoder with open collector output will thus be
sufficient.
Actual counter value
You can read the actual counter value in parameter
544 to check whether the frequency converter
registers the correct number of pulses on the pulse
input.
During commissioning it is also often useful to know
the counter value, for example, if mechanical
adjustments are based on the counter value.
Parameter 544 can be programmed into the user
defined quick menu so that the counter value can
be read directly from the frequency converter
display. Please consult the VLT 2800 Design Guide
for further information about how to program the
user defined quick menu.
With this function the ramp should also consist of at
least 10 cycles (130 ms).
Counter with or without reset
The desired counter value is programmed in
parameter 344. The maximum value is 100000.
You can choose whether you want the counter to
reset so that it starts from zero each time. If you
choose without reset the overrun value is stored in
the memory of the frequency converter and next
time the counter starts from this value. You can for
example use this function to compensate for the
extra distance done during ramping down and to
reduce the impacts of gradual wear of mechanical
parts.
PLC counter module
One of the advantages of using the built-in counter
is that a simpler and cheaper PLC without high
frequency input can be used.
Via the digital output the VLT can inform the PLC
whether it runs or has stopped. Such information is
for example required if one movement must be
synchronised with other events.
Choice of encoder
Different types of encoders are available. Primarily
we distinguish between absolute and incremental
encoders. Absolute encoders give a specific digital
value for each individual position of the shaft,
whereas an incremental encoder gives a number of
pulses per revolution (typically 512, 1024 or 4096 per
revolution).
GuidelinesGuidelines
GuidelinesGuidelines
Guidelines
In this section we go through the factors to be
considered for counter stop applications.
Start/stop and ramps
As counter stop works with the precise stop
function the VLT 2800 must be programmed for
precise start/stop as start input for the digital inputs
(terminal 18 or 19). The start signal must be active
on the digital input during ramping and be removed
before the end value is reached.
The output signal from an encoder is either of an
open collector or of a push-pull type. Open collector
outputs can be used up to 20 kHz with VLT 2800
frequency converters. If the pulse signal has a
higher frequency it is necessary to use a push-pull
output. The VLT 2800 accepts pulse inputs up to
67 kHz. The voltage level is either 5 VDC (TTL
levels, line driver) or 10-30V DC (HTL levels). For VLT
2800 encoders with HTL levels must be used.
The minimum frequency, the VLT 2800 can detect
on the pulse input, is 5 Hz.
VLT 2800 Series - Precise Stop
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Calculation of motor size
The output power available to accelerate the load is
often the limiting factor when motors are used in
applications with frequent starts and stops. The
frequency converter limits the applied output to
160% of rated motor output. However, a special
high torque mode can be programmed so that the
torque can be 180% for a short period of time (up to
0.5 s). With the VLT 2800 we have made this
limitation to protect the output of the frequency
converter against overload and the motor against
reaching the pull out torque.
When the frequency converter reaches the current
limit it will limit the ramp and try to ramp up at a
lower speed. This means that the conveyor belt
does not reach the required speed within the
programmed time. In connection with speed
compensated stop this can give rise to faults.
Contrary to expectation it could in such cases help
to increase the ramp time to make the belt reach
the expected speed, without reaching the current
limit.
For the time t
0
the belt will travel the distance:
S
0
=
V
max
.
t
0
2
The ramp of the frequency converter is limited so
that it will need a ramp of t
1
= k
.
t
0
to reach V
max
,
where k is a constant. This is illustrated in Figure 13.
The stop signal will come at the same distance as
the proximity sensor position is fixed. The
acceleration of the belt with the new ramp is:
a
=
V
max
=
V
max
t
1
k
.
t
0
The distance done at any time, t, during ramp up
can then be calculated as:
Figure 13Figure 13
Figure 13Figure 13
Figure 13 Current limited ramp
S
1
=
1
a
.
t
2
=
1 V
max
t
2
2 2 k
.
t
0
The proximity sensor is reached when S
0
= S
1
:
1 V
max
t
2
=
V
max
.
t
0
t
1
= k
.
t
0
2 k
.
t
0
1
2
And the speed at t
1
will be:
v
1
= a
.
t
1
=
V
max
k
.
t
0
=
V
max
k
.
t
0
k
Appendix A
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MI.28.C1.02 - VLT is a registered Danfoss trademark
12
ExampleExample
ExampleExample
Example:
If the ramp is extended to
the double the speed will
only be 71% of the expected speed when the work
piece passes the proximity sensor, which is a
significant deviation.
In the following we go through how the necessary
motor output can be calculated in a typical
horisontal conveyor belt application. The distance
done and the material on the conveyor belt will vary
from application to application. The ramp time is
typically determined by the processing time, e.g.
expressed as a cycle time, whereas the maximum
belt speed is often limited by mechanical factors.
The conveyor belt has a total length of L, and
between the rolls there is a distance of dL. Each roll
weights M
r
, and has a radius of R. The total mass
on the conveyor belt is M and the belt/mass must
move the distance L
0
within the time t
0
, see Figure
14.
The frequency converter output to the motor/gear
shaft is: η
.
U
.
I, where η is the total motor and
gear efficiency, η = η
motor
. η
gear
.
The energy delivered to the conveyor belt during
acceleration is:
E
out
= η
.
U
.
I
.
dt = η
.
U
.
I
.
t
0
,
0
2
Max. speed belt, acceleration, force on the box,
and the maximum angular velocity of the rolls can
be calculated as follows:
t
0
2
FigurFigur
FigurFigur
Figur
e 14e 14
e 14e 14
e 14 Typical conveyor belt application
V
max =
2L
0
;a =
4L
0
;
t
0 t
2
0
F = M
.
a =
4L
0
M
; ω
max
=
V
max
t
0
R
2
VLT 2800 Series - Precise Stop
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E
mass
+ E
conveyor
+ E
motor
= (P
mass
+ P
conveyor
+ P
motor) dt
0
= (F + F
fric
) ⋅ R ω + I
conveyor
ω ω + I
motor
⋅ω ω dt
0
= 4
ML
0
R
ω
max
t
+
1
+M
r
R
2
+ I
motor
ω
max
ω
max
t
dt
0
t
2
t
0
2 t
0
t
0
0
= 8
ML
0
2
+
1
+M
r
R
2
+ I
motor
ω
max
ω
max
t
1
t
2
t
0
/2
t
0
2 t
0
t
0 2
0
=8
ML
0
2
+
1
+M
r
R
2
+ I
motor
4
L
0
2
1
t
2
t
4
2
R
2
t
4
8
0
0
0
=
ML
0
2
+
1
M
r
+
I
motor
1 L
0
2
t
2
2
R
2
2 t
0
0
To accelerate the belt and the material on the belt
the following energy is required:
tt
tt
t
00
00
0
//
//
/
22
22
2
tt
tt
t
00
00
0
//
//
/
22
22
2
tt
tt
t
00
00
0
//
//
/
22
22
2
4
As E
out
= E
mass
+ E
conveyor
+ E
motor
, and the
frequency converter can supply 160 % torque for up
to 60 s. the following calculation can be made:
As it will appear from the above the necessary
output power is highly dependent on the ramp time,
t
0
.
Example:Example:
Example:Example:
Example:
A conveyor belt of 2 m, a roll distance of 10 cm and
a roll radius of 2.5 cm and a mass of 200 g per roll
transports a mass of 5 kg over a distance of 20 cm
within a given time. If it must be done as described
above in Figure 1 with a total motor and gear
efficiency of 0.85 and a moment of inertia of the
E
out
= E
mass
+ E
conveyor
+ E
motor
1.6 η U I
t
0
=
ML
0
+
1
M
r
+
I
motor
1
L
0
2
2 t
2
2 R
2
2 t
0
0
U
I = P
nom
=
1 L
0
2M +
L
M
r
+
I
motor
1.6 η t
3
2dL R
2
0
motor and gear of 0.02 kgm
2
,
the frequency
converter must supply the motor with an output
power as shown in Figure 15. It can be seen that
the output is very high for short ramps compared to
the output you would normally expect for such a
conveyor belt in steady state.
2
2
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14
The frequency converter may thus reach the
current limit during acceleration if for instance a 0.75
kW frequency converter and motor are used. One
solution is to choose a frequency converter one size
larger than the motor to enable a higher power.
Caution should be paid not to exceed the pull out
torque of the motor.
Figure 15 Figure 15
Figure 15 Figure 15
Figure 15 Output required to accelerate a conveyor belt
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Danfoss VLT 2800 (Legacy Product) Installation guide

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Installation guide
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